1. Field of the Invention
The present invention relates to a high temperature anticorrosive molded product which is prepared by thermal compression (hot press) of powder comprising an electrically conductive refractory, boron nitride (BN), aluminum nitride (AlN) and aluminum (Al). This product can be used at high temperatures and is suitable for use in the preparation of crucibles, boats or the like which are commonly used for melting or vacuum evaporating high temperature corrosive metals such as aluminum.
2. Description of the Prior Art
Heretofore, it has been proposed to use an electrically conductive anticorrosive refractory, such as TiB.sub.2, ZrB.sub.2 , and the like in order to prepare electrical heating containers for use in evaporation of metals, e.g., aluminum. Aluminum nitride (AlN) decomposes at high temperatures by the following reaction: EQU 2AlN .fwdarw. 2 Al + N.sub.2 .uparw. (1)
as a result of the formation of the highly volatile nitrogen gas, it has been difficult to obtain dense products.
In order to adjust only the resistance of the electrically conductive refractory, it is possible to add BN or AlN to the refractory. When BN is added to the refractory, the TiB.sub.2 -- BN system forms. When using these systems as containers for evaporation of Al, the aluminum reacts with BN as follows: EQU 2.83 BN +3 Al .fwdarw. 2.83 AlN + 0.17 AlB.sub.12 + 0.79 B (2)
the result is the formation of AlB.sub.12 and B, which possess inferior anticorrosive properties and heat resistance. On the other hand, when AlN is added to the refractory, TiB.sub.2 -- AlN is formed and the resulting molded product will be characterized by low thermal shock resistance. The resulting product can thus not be used as a container for vacuum evaporation or the like, particularly where rapid heating and cooling may occur. The specific resistances of TiB.sub.2 and ZrB.sub.2 used in the electrically conductive refractory of this invention are, respectively, 14.4 .mu..OMEGA.-cm and 16.6 .mu..OMEGA.-cm. (Plenum Press Handbook of High Temperature Materials, Plenum Press, New York, 1964).
In order to prepare a molded product having a specific resistance of 100- 2000 .mu..OMEGA.-cm, as is required for heating containers, such as crucible boats used for vacuum evaporation, it is necessary to add 20-70 wt. % of an electrically insulating anticorrosive material, such as BN, AlN, etc. When 20-70 wt. % of only BN or AlN is added to the refractory, the above-mentioned disadvantages result. In addition, even though a combination of BN and AlN is added to the refractory, decomposition of AlN at the high temperatures used in the molding operation occurs and it is difficult to obtain a dense product and low anticorrosive properties when in contact with molten metals. The inventors have studied the above-mentioned findings and have found that these disadvantages can be overcome by adding a combination of AlN, BN and Al to the electrically conductive refractory.